Polyynes and Other Products from Laser-Ablated Graphite: A Time-of-Flight Mass Spectroscopic Study in Combination with One-Photon Ionization - Polyynes: Synthesis, Properties, and Applications - page 180

Products ionized by the 10.5 eV photons were accelerated with static
electric fields created by three plates whose voltages were kept at þ 4.80 kV,

þ 3.98 kV, and 0 V, respectively, as shown in The distances

between the first and second electrodes and the second and third electrodes
were 2 cm and 1.5 cm, respectively. The accelerated ions were detected by
MCPs (multi-channel plates, Galileo 6205) placed at 75.3 cm downstream
from the ionization point. The signals detected by the MCPs were recorded
with a digitizing oscilloscope (Tektronix TDS544A) and plotted as a func-
tion of time to obtain TOF mass spectra. All the measurements were carried
out at a repetition rate of 10Hz.
The time sequence of the ablation pulse, the ionization pulse, and the
recording of TOF signals by the detector is shown in the lower part of
Figure 9.1. Spectra were recorded for various time intervals between the
ablation and ionization pulses. These time intervals are called delay

Figure 9.1 .

times as stated in Section 9.2 (see Figure 9.1). The timings of all the instru-

ments were controlled by delay generators (Stanford Research Systems
DG535).

9.4 RESULTS

9.4.1 C OMPARISON OF O NE -P HOTON AND M ULTI -P HOTON
I ONIZATION IN THE He B UFFER G AS

In order to investigate intact neutral ablated products one has to ionize the
molecules as gently as possible to avoid secondary decomposition by laser.
In this sense, often used MPI techniques are not suited, as mentioned briefly
in Section 9.2; intense electric fields of MPI pulses tend to cause extensive
fragmentation of the ablated products. Moreover, accidental resonance
enhancement of some specific mass peaks may distort the true mass distri-
bution pattern.
An example of the difference in the effect of one-photon ionization (1PI)

and MPI can be seen from the two mass spectra in Figure 9.2 , which were

obtained by using the ninth harmonics of a Nd:YAG laser (118 nm ¼

10.5 eV/photon) for 1PI and by using the third harmonics of a Nd:YAG
laser (355 nm

3.5 eV/photon) for MPI in the He buffer gas.
The energy of 10.5 eV/photon can ionize all the carbon clusters C n with

¼

n 6 by a single photon, as is shown in Figure 9.3 (see also Figure 9.2 of

Reference 17 ) . The TOF mass pattern obtained by 1PI showed an intensity

distribution of products peaking at about the cluster size n of C n equal 10.
Larger clusters up to C 80 were detected, but the signal intensities of the
larger carbon clusters were 100 times smaller than the signal of C 10 .Itis
noted that both even-numbered clusters and odd-numbered clusters were
detected as the products of larger n values, as is seen in the inset of the right